This invention relates to injection molding machines, and more particularly, to an improved method and apparatus for forming a molded article and removing the same from the mold.
The molding of articles having outer and inner peripheral features such as threads, lips, tabs or the like, is well known in the molding industry. Generally, a female mold and a mold core are used for molding such articles wherein molten plastic or the like is injected between the core and female mold for creating the desired configuration. Once molding has taken place, it is a necessary feature of injection molders to have an ejection mechanism which removes the molded article from the mold. Because molds generally include a large number of cavities, it is frequently the case that the ejection mechanism operates in some manner to eject the articles as a group.
A particular use for ejection mechanisms is derived from the manufacture of molded closures having threads on the inner surface thereof. In these processes, the female mold is configured to form the outer portion of the closure while the mold core is configured to form the inner portion of the closure which includes a threaded surface. Once the article is molded, the female mold and mold core are separated so as to initiate the release of the article from the mold. Because of the engaging nature of threads, the article stays with the mold core upon separation of the mold halves. Accordingly, a mechanism is typically required which functions with the mold core for removing the threaded molded article from the outer surface of the mold core.
The art includes a variety of devices and methods for removing threaded closures or the like from a mold. A common denominator between most of the devices discussed below, is the necessity for an initial force for breaking or loosening the article away from the mold which is generally significantly larger than the force required for finally ejecting the article. Accordingly, a common disadvantage of ejection devices and methods currently available, is the fact that large sources of power are necessary for initiating the ejection process which is then used inefficiently thereafter for finishing the ejection process. As a result, a large force is used for the entire ejection process where only the initiation of the process requires such a large force. The requirements for large forces naturally requires larger devices for providing these forces. As such, excessively large hydraulic cylinders or other means for power supply are used in these devices.
The art of injection molding plastic closures is well known in the industry. Dependant upon the design of the plastic closure, including the thread type and number of complete threads, it may be ejected from the mold in a variety of ways. If the plastic material being molded is flexible or resilient it may be stripped by a commonly known stripper ring pushing the article off the core. The molded part must be solidified enough not to be folded over onto itself during ejection but sufficiently elastic to return to essentially its original molded shape after the threads have been stretched over the core. If the molded material does not possess the appropriate characteristics of flexibility for this most economical and simple method of ejection, the article will be damaged or may not return to its original shape and size. Similarly, a very defined or deep thread profile may be inherently prone to stripping damage. Also, the part may have other delicate or fragile features such as a commonly known tamper evident ring, which could be damaged even if an otherwise acceptable plastic is being injection molded.
When molding a plastic resin which is too rigid to be stripped from the threaded portion of the cavity, without permanent deformation of the molded article, one may use a method as shown in U.S. Pat. No. 4,652,227. In this patent, an unscrewing chuck is used to come over the molded part once the female portion of the cavity is removed. The chuck grasps the outer feature of the closure, then rotates the closure as it moves slidably backward to facilitate the axial movement of the unscrewing closure. This method has the advantage of a simpler mold design which requires no moving parts for ejection of the article; however, the unscrewing system can become quite complicated and costly as it is virtually a machine unto itself. The chuck is actuated by an electric motor which axially moves and rotates the chuck. The power of the electric motor is essential for initially breaking the article from the mold. However, such power is unnecessary for the remaining ejection process. Also, the electric motor takes up a large amount of room adjacent the molding device and thereby substantially limits the number of mold cavities which can be used.
Another method of unscrewing is accomplished by first separation of the mold halves (opening the mold), then rotating the core portion of the mold cavity while a stripper ring or bottom forming portion of the cavity moves slidably forward in timed relationship with the axial movement of the closure, until the closure is clear of the core portion and allowed to fall free from the molding area. The stripper ring will often employ protruding features, such as teeth or notches, which will keep the molded article from turning in the direction of the rotating core. The stripper ring may be moved forward by a sliding cam attachment that is linked to the rotation drive of the core, thereby ejecting the closure from the core.
This approach carries with it various disadvantages and short-comings. If the core rotation and stripper ring plate movement are directly linked to a motive means which in turn is not directly linked to the mold closing means, there is the likelihood of collision damage should the mold closing means complete its task before the stripper ring return means. To eliminate this possibility, the stripper ring return means must be completed in sequence before the mold closing begins. These non-simultaneous movements can undesirably add one or two seconds to the entire mold cycle time. In addition, the rotation drive must be of sufficient power to break the molded article from the mold. Thus, as discussed above, a large initial force is required in comparison to a smaller finishing force. With one rotation device turning the core and stripper ring, typically one excessively large force is used inefficiently for the finishing portion of the ejection process.
There exists, therefore, a need in the injection molding field, for an ejection device having an initial power source for supplying a large force required for breaking a molded article loose of its mold, which is then replaced by a less powerful source for completing the ejecting process.